Graphene sheets (GS), sp2-hybridized, two-dimensional (2D) carbon monolayers, are attracting intense interest owing to their inherent exciting physical and electronic properties. Graphene flakes is another term used for graphene sheets, and both terms are used alternatingly in this application.
Various methods of making such materials including exfoliating graphite by applying sonication, cracking, milling, and grinding are known in the art.
Direct exfoliation of graphite particles has the potential of becoming the most economical manufacturing method for producing large quantities of single layer graphene. However, the tight interlayer spacing of graphite and strong cohesive forces between the layers make direct exfoliation of graphite extremely difficult. Mechanical exfoliation either through shear forces or through sonication has only met limited success.
Graphene sheets can be produced through the exfoliation of graphite, a widespread mineral in nature, by mechanical cleavage, ball-milling, or direct exfoliation in liquids. Liquid-exfoliation of graphite through the sonication of graphite suspensions in specific liquid environment, such as Dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP) has been attracting widespread interest since it was firstly reported by Coleman et al. in 2008, for its simplicity and cost-effectiveness. See for example, Hernandez, Y.; Nicolosi, V.; Lotya, M.; Blighe, F. M.; Sun, Z.; De, S.; McGovern, I. T.; Holland, B.; Byrne, M.; Gun'Ko, Y. K.; Boland, J. J.; Niraj, P.; Duesberg, G.; Krishnamurthy, S.; Goodhue, R.; Hutchison, J.; Scardaci, V.; Ferrari, A. C.; Coleman, J. N., High-yield production of graphene by liquid-phase exfoliation of graphite. Nat Nano 2008, 3 (9), 563-568; Khan, U.; Porwal, H.; O'Neill, A.; Nawaz, K.; May, P.; Coleman, J. N., Solvent-Exfoliated Graphene at Extremely High Concentration. Langmuir 2011, 27 (15), 9077-9082; and Barwich, S.; Khan, U.; Coleman, J. N., A Technique To Pretreat Graphite Which Allows the Rapid Dispersion of Defect-Free Graphene in Solvents at High Concentration. The Journal of Physical Chemistry C 2013, 117 (37dsa), 19212-19218.
However, it is found that the affinity of such solvent molecules to graphene is too weak to overcome the strong van der Waals force between the π-π stacked layers of graphite. As a result, the liquid exfoliation process is superficial and slow. Graphite surface roughening and exfoliation were achieved only after hours of sonication. Other exciting advances suggest that shear should be used for liquid exfoliation of graphite into graphene. Nevertheless, these methods still encounter one major challenge: their relatively low efficiency. Typically, ˜0.1 mg/ml of graphene could be produced after having applied hundreds of hours of bath-sonication or hours of shear (if applying relatively rigorous centrifugation (>1,000 g) to remove the un-exfoliated large flakes). Exfoliating graphite in water/surfactant, polymer solution and ionic liquids were also reported as promising approaches, but washing-off the residual molecules from graphene flakes was necessary after the exfoliation step to alleviate negative impact on the electrical and mechanical performances of GS-based devices by such molecules.
Also known in the prior art is Chang et al., U.S. Pat. No. 9,327,984 B2, issued May 3, 2016, which is said to disclose a method for preparing graphene nanoplate (GNP) that includes preparing expanded graphite (EG) and exfoliating, grinding, or cracking the expanded graphite to crack the EG induced by gas-phase-collision. A graphene nanoplate paste and a conductive coating layer formed of the graphene nanoplate paste are provided and are prepared by the method for preparing graphene nanoplate.
There is a need for improved processes for producing graphene material.